1. Field of the Invention
The present invention relates to a handset for a communication unit, and more particularly to a handset for a communication unit such as a telephone set, a cell phone and like communication units each provided with a bone conduction speaker and a microphone.
2. Description of the Related Art
In general, a conventional handset for a communication unit has a construction such as one shown in FIG. 4. More specifically, the handset has a casing 11′. The casing 11′ is provided with a speaker portion 12′ and a microphone portion 13′ in its upper end portion and its lower end portion, respectively. A speaker and a microphone are incorporated in the speaker portion 12′ and the microphone portion 13′ of the handset, respectively. In use, a user has the speaker portion 12′ of the handset brought into contact with his or her ear and has the microphone portion 13′ of the handset disposed in the vicinity of his or her mouth to communicate with his or her partner in communication.
When a user normal in the sense of hearing uses the conventional handset having the above construction, there is no need for such a normal user to excessively increase an output of the speaker in sound pressure level. In this case, therefore, there is no fear that a so-called “howling” problem occurs. In contrast with this, when a user poor in the sense of hearing uses the conventional handset, he or she must increase the output of the speaker in sound pressure level to compensate for his or her poor hearing ability. This often causes the howling problem between the speaker and the microphone both housed in the same casing of the conventional handset. Particularly, the howling problem is very often recognized in the conventional handset employing a bone conduction speaker as its speaker component.
The reason why the conventional handset employing the bone conduction speaker often suffers from the howling problem is that: a voice sound to be transmitted is transmitted through kinetic vibrations in the bone conduction speaker; a face-abutting surface 14′ (shown in FIG. 4) of the casing 11′ in the conventional handset serves as a vibrating plate or diaphragm to enhance the kinetic vibrations; and, the microphone is mounted on the same face-abutting surface 14 of the conventional handset, and therefore effectively picks up the kinetic vibrations there of to often cause the howling problem. Further, in order to obtain a sufficiently large output of the bone conduction speaker, it is necessary to apply a sufficiently large input on the bone conduction speaker. This also increases the possibility of occurrence of the howling problem in the conventional handset employing the bone conduction speaker.
Further, when the user covers or grasps the speaker portion 12′ and/or the microphone portion 13′ of the conventional handset with his or her hand in use, the output of the speaker is reflected at both the user's hand and the casing of the conventional handset. As a result, the sound pressure level applied to the microphone often reaches a value of 20dB or more at maximum to cause the howling problem. This is shown in a diagram of FIG. 5, wherein: a kinked-line graph “A” shows, in value, the sound pressure level applied to the microphone which is substantially free from any contact; and, another kinked-linegraph “B” shows, in value, the sound pressure level applied to the microphone housed in the microphone portion 13′, which is covered with the user's hand.